Conventionally, heavy petroleum fractions such as gas oils, vacuum gas oils, coker gas oils, and the like, have been advantageously cracked in the presence of a catalyst to produce lower boiling petroleum fractions including a high octane gasoline fraction. Conventionally, catalytic cracking units are operated at conditions so that a fraction or fractions boiling at a temperature above the distillation range of gasoline are also recovered from the catalytic cracking zone. The gasoline and lower boiling materials are conventionally separated by fractionation from such higher boiling product fractions.
Typically, a fraction recovered from a catalytic cracking process boiling higher than the gasoline fraction is identified as a cycle oil which can comprise light and heavy cycle oils and a slurry oil. The heavy cycle oil and slurry oil are normally recycled to the catalytic reaction zone. The refractory light cycle oil fraction due to the high polynuclear aromatic content thereof cannot be economically recycled to the catalytic reaction zone.